The present invention relates to a method of manufacturing high-strength, high-density carbon materials from specific powdered carbonaceous raw materials without use of a binder.
Also, the present invention concerns a method of manufacturing high strength carbon materials reinforced with specific carbon fibers.
Among various carbon materials, high-strength, high-density carbon materials are used in a wide variety of applications, such as for electrodes, carbon materials for use in nuclear reactors, crucibles, heating elements, mechanical seals, sliding members, current collecting elements, and dies for hot pressing.
These high-strength, high-density carbon materials have conventionally been prepared by an extremely complicated and time-consuming technique which comprises the steps of finely pulverizing a carbonaceous material such as coke or graphite to a powder having an average particle diameter of less than 10 .mu.m, hot kneading the powder with a binder such as coal tar pitch, re-pulverizing, shaping and baking the kneaded material and impregnating the baked body with coal tar pitch or a similar binder followed by re-baking wherein the last two steps of impregnation and re-baking are carried out repeatedly until the bulk density of the resulting carbon material reaches approximately 1.8.
The above-described prior art technique has many problems in addition to its complexity. For example, micropores are formed due to the difference in shrinkage factor between the filler, such as coke or graphite, and the binder, the pores inherently present in the filler remain after baking, additional pores are formed by vaporization of volatile substances contained in the binder, and oxidation of the binder tends to make graphitization difficult to some extent. For these reasons, the manufacture of high-strength, high-density carbon materials is difficult and their manufacturing costs are considerably high.
Various attempts have been made in the art to obtain less expensively high-strength, high-density carbon materials of good quality, e.g., those having a strength of 700 kg/cm.sup.2 or higher and a bulk density of 1.9 g/cc or higher.
It has been proposed in Japanese Patent Publication No. 53-18359 that a carbonaceous raw material having a specific hydrogen-to-carbon atomic ratio, quinoline-soluble content, degree of heat distortion shrinkage and degree of carbonization be finely pulverized to an average particle diameter of less than 10 .mu.m, then shaped and baked for carbonization and graphitization to produce a high-strength, high-density carbon material.
Japanese Laid-Open Patent Application No. 56-22615 discloses a method of preparing a carbonaceous raw material suitable for use in the manufacture of high-strength, high-density carbon materials, which comprises heat-treating a petroleum-derived heavy oil or coal tar pitch and isolating the resulting optically anisotropic microspheres or mesophase particles by solvent precipitation fractionation.
Both of these methods aim to manufacture high-strength, high-density carbon materials in the absence of a binder by using a carbonaceous raw material which serves not only as a filler but as a binder and hence has a self-sintering property, and which is therefore effective for preventing the formation of cracks and pores which may occur during baking.
In the method disclosed in the above Japanese Patent Publication No. 53-18359, a carbon precursor or precarbon which is a porous coke-like carbonized material comprising flow texture phases interspersed with mesophases and which exhibits optical anisotropy as a whole is used to manufacture a high-strength, high-density carbon material and is mechanically pulverized to an average particle diameter of 10 .mu.m or less. Such pulverization, however, requires a special mill whose operation is time-consuming and expensive. In addition, the use of finely pulverized powder as above does not permit air entrapped in the powder to escape fast enough for the rate of shaping to be increased to a satisfactory level, and the pores through which the gas evolved during baking escapes to the surroundings are also small. Therefore, a considerable pressure develops inside the shaped body during baking, and cracking tends to occur in the resulting baked article.
According to the method disclosed in the aforementioned Japanese Laid-Open Application No. 56-22615, the optically anisotropic microspheres isolated by solvent precipitation fractionation contain internal cracks formed by extraction of some component with the solvent and these internal cracks remain in the resulting carbon material manufactured by shaping and baking the microspheres. It is difficult, therefore, to obtain high-strength, high-density carbon materials using this method.
With respect to high strength carbon materials suitable for use in the above-mentioned applications, it is known to incorporate a carbon fiber reinforcement in carbon products in order to further improve the mechanical and other properties.
Such carbon materials reinforced with carbon fibers have been conventionally manufactured by impregnating a woven fabric of carbon fibers with a thermosetting polymer such as a phenolic resin and, after curing the resin, carrying out baking for carbonization and/or graphitization. In order to further increase the density by filling the pores evolved during baking with the thermosetting resin, the resin impregnation and baking are usually repeated four or five times.
According to such a prior art technique, expensive carbon fibers are incorporated in a high proportion and many steps are involved to obtain the desired product. Thus, the resulting reinforced carbon materials are extremely expensive so that they can be used in limited industrial fields such as the aircraft and space industries and are not suitable for use in common industries.
In order to minimize the number of steps, it has been proposed to manufacture carbon fiber-reinforced carbon material without an impregnation procedure. For example, Japanese Patent Publication No. 49-29281 discloses a method in which specific organic fibers are added to an inorganic or carbon filler and an organic binder before baking. Japanese Laid-Open Patent Application No. 51-87515 discloses a method comprising kneading a carbon powder and a binder with carbon fibers (including graphite fibers) the surfaces of which have been wetted with a liquid carbonaceous binder, followed by shaping and baking.
Although the carbon fibers employed in these methods are specially defined or treated so as to adapt them to use without need of impregnation treatment, there are many problems with these methods. In the method of Japanese Patent Publication No. 49-29281, the inorganic or carbon filler in the form of coarse particles causes the valuable carbon fibers to break into powders during kneading and shaping procedures. According to the method of Japanese Laid-Open Patent Application No. 51-87515, it is essential to thoroughly wet the surfaces of the individual carbon fibers with a carbonaceous binder before kneading, and many microcracks are formed in the baking stage because of different shrinkage factors of the carbon fibers, carbon powder, and organic binder. For these reasons, even in the case of carbon fiber-reinforced carbon materials, a product having a flexural strength of higher than 700 kg/cm.sup.2 can not be obtained by any method known in the art.